The flow rate computer adjunct relates to conversion of impedance plethysmograph information to usable clinical data, and more particularly to the transposition of biological segment conductance, impedance deviation, and heart pumping rate signals to an output signal indicating blood flow rate which is relatively free of artifact errors.
Impedance plethysmography is a non-invasive, analog analysis of body mechanical activity which depends upon the electrical prpoerties of the body tissue. An impedance plethysmograph and flow rate computer adjunct and method is described in U.S. Pat. No. 3,835,839, assigned to the assignee of the instant invention. The combination disclosed therein includes a DC restorer for returning the signal indicative of deviation from biological segment basic resistance to a zero reference at the beginning of each deviation cycle. This technique was utilized in an attempt to remove artifact errors from the deviation signal. While some error was removed thereby, a large error still remained and output signal stability was such as to severely limit the use thereof.
As further described therein tissue segment blood flow rate determination has previously been accomplished by graphically recording plethysmograph output. A wave shape proportional to the deviation in biological segment volume, and thus flow rate, was used. Blood flowing through the arterial system into the segment flows away from the segment due to venous runoff. Since runoff is occurring simultaneously with arterial inflow, the entire volume of blood pumped through the segment during one heart pumping cycle is not present in the segment at one time. Therefore, the graph of volumetric deviation within the segment does not directly indicate the maximum volume of flow through the segment for each heart pump cycle. The volume of flow within the segment builds up relatively rapidly at the beginning of the pumping cycle and then begins to trail off as venous runoff begins to occur. There is a primary slope on the trailing edge of the graphical recording of the pumping cycle which has an average negative slope. Extending the average negative slope to a point where it intersects the ordinate drawn through the beginning of the pulse, provides an extrapolated value which theory and practice have shown to represent a quantity proportional to the true volumetric flow through the segment for each heart pumping cycle. Attempts other than that described in U.S. Pat. No. 3,835,839 mentioned above have been made to perform the graphical extrapolation electronically. These attempts have proven unsatisfactory because of interfering noise levels and, in the apparatus and method disclosed in the above-referenced patent, because of the presence of artifact signals as discussed above. There is, therefore, a need for an apparatus and method for measuring blood flow rate through a biological segment which provides a stable output signal and reduction of noise induced output signal error.